Embodiments of the invention relate generally to x-ray tubes and, more particularly, to an apparatus for wide coverage computed tomography and a method of constructing same.
Computed tomography X-ray imaging systems typically include an x-ray tube, a detector, and a gantry assembly to support the x-ray tube and the detector. In operation, an imaging table, on which an object is positioned, is located between the x-ray tube and the detector. The x-ray tube typically emits radiation, such as x-rays, toward the object. The radiation typically passes through the object on the imaging table and impinges on the detector. As radiation passes through the object, internal structures of the object cause spatial variances in the radiation received at the detector. The detector converts the received radiation to electrical signals and then transmits data received, and the system translates the radiation variances into an image, which may be used to evaluate the internal structure of the object. One skilled in the art will recognize that the object may include, but is not limited to, a patient in a medical imaging procedure and an inanimate object as in, for instance, a package in an x-ray scanner or computed tomography (CT) package scanner.
A typical x-ray tube includes a cathode that provides a focused high energy electron beam that is accelerated across a cathode-to-anode vacuum gap and produces x-rays upon impact with an active material or target provided. Because of the high temperatures generated when the electron beam strikes the target, typically the target assembly is rotated at high rotational speed for purposes of cooling the target.
As such, the x-ray tube also includes a rotating system that rotates the target for the purpose of distributing the heat generated at a focal spot on the target. The rotating subsystem is typically rotated by an induction motor having a cylindrical rotor built into an axle that supports a disc-shaped target and an iron stator structure with copper windings that surrounds an elongated neck of the x-ray tube. The rotor of the rotating subsystem assembly is driven by the stator.
Computed tomography systems are continuing to increase the size of the detector along a patient body or Z-axis, so that entire organs (e.g., heart, brain) can be imaged in one rotation of the gantry. Consequently, a vertical opening angle of the x-ray transmissive window in the x-ray tube is typically broadened further to allow for irradiation of the examination object covering a wider extent along the Z-axis. As z-axis coverage increases, a fan out of the x-ray beam along the wider z-axis can cause image artifacts or so-call cone-beam artifact due to missing data in the image reconstruction. Additionally, a target angle of the x-ray tube is typically increased to accommodate the larger vertical opening angle and fully cover the detector. This larger target angle significantly reduces an amount of x-ray flux that can be generated due to temperature limitations on the rotating target (holding the optical focal spot size constant as determined by the well-known line focus principle).
A theoretical solution to these problems is to provide two or more focal spots spaced apart from each other along the z-axis and operating in an on/off sequential manner. This optical configuration can allow for an overlap of x-ray beams from each of the focal spots over some extent of the center of view and achieve a reduction in the cone-beam angle, thereby, greatly decreasing cone-beam artifacts. This configuration also allows for a reduction in the target angles and consequently higher x-ray output. One possible solution to the problem is to make an x-ray tube with two targets instead of only one target such that the two focal points are offset along the z-axis or axial dimension. The difficulty with that solution is, however, that the use of two axially offset targets may require an inordinately large and complicated x-ray tube. That is, such a tube may have two targets in one vacuum cavity, two cathodes and two high voltage insulators, one on each end to feed the cathode. Such a tube would be very costly and have reliability implications, that is, if one of the cathodes or targets were to fail, the entire tube would need to be replaced to maintain full functionality, resulting in a costly replacement.
Another solution to the problem would be to abut or configure end to end two conventional x-ray tubes, however, typical CT x-ray tubes are simply physically too lengthy to make that solution practical. With the present x-ray tubes, the tubes include a cathode, insulator and other systems for focusing the high energy electrons onto the target, and that apparatus is located facing the target face of the rotating anode. At the same time, the rotating system for rotating the anode is also cumbersome and that system is located facing the opposite side, or non-target side surface of the anode. Together, two conventional x-ray sources placed end-to-end along their respective rotational axes would space the focal spots too far from each other to make a useable source for the desired system configuration. For example, a desired axial spacing on the z-axis in a two focal spot system is 120 mm coverage at the system iso-center or typically in the range of 60-120 mm. Therefore, the advantages derived from the presence of two focal points is not feasible with conventional x-ray tubes.
Accordingly, it would be advantageous to have an x-ray tube that could be abutted together with another x-ray tube and achieve a spaced apart distance for the focal points that is within the desired range and thereby gain the advantage of using a combined apparatus with two x-ray tubes and two focal points.